Journal of the Korean Physical Society, Vol 61, No 10, November 2012, pp 1568∼1572 Influences of Ru-doping on the Magnetic Properties of Ca0.85 Pr0.15 Mn1−x Rux O3 T L Phan, Y D Zhang and S C Yu∗ BK-21 Program and Department of Physics, Chungbuk National University, Cheongju 361-763, Korea P Q Thanh and P D H Yen Department of Physics, College of Science, Vietnam National University, Hanoi, Vietnam (Received December 2011) CaMnO3 is an antiferromagnet, in which the super-exchange interaction taking place between Mn4+ ions plays an important role The doping of a small amount of 15% Pr into the Ca site, Ca0.85 Pr0.15 MnO3 , leads to the appearance of Mn3+ ions, and introduces the ferromagnetic (FM) double-exchange interaction between Mn3+ and Mn4+ ions, which is dominant in a narrow temperature range of 90 ∼ 115 K The FM interaction becomes strong for Ca0.85 Pr0.15 MnO3 doped with and 8% Ru into the Mn site (i.e., Ca0.85 Pr0.15 Mn1−x Rux O3 with x = 0.04 and 0.08) The Curie temperature obtained for x = 0.04 and 0.08 are about 135 and 180 K, respectively While the FM interaction in the former is dominant due to Mn3+ -Mn4+ exchange pairs, the latter has the contribution of Ru ions This results in remarkable differences in the features of their FM-paramagnetic phase transitions and their coercive fields Hc PACS numbers: 75.30.Kz, 75.47.Lx, 75.60.Nt Keywords: Ru-doped perovskite manganites, Magnetic properties DOI: 10.3938/jkps.61.1568 I INTRODUCTION Recently, CaMnO3 -based perovskite manganites have received much interest from the solid-state physics community because of their showing interesting electrical and magnetic properties around their phase-transition temperatures [1–5] In the parent compound of CaMnO3 , the formal valence of Mn is 4+, and the magnetic interaction between Mn4+ ions is antiferromagnetic (AFM) superexchange; its N´eel temperature is 125 K [5] The presence of intrinsic defects due to insufficient oxygen (i.e., CaMnO3−δ ) leads to the appearance of Mn3+ , which stimulates a ferromagnetic (FM) double-exchange interaction between Mn3+ and Mn4+ ions Random competing AFM and FM interactions can cause magnetic frustration as in spin-glass or mictomagnetic systems [5] For the case of sufficient oxygen content, Mn3+ ions can be generated by doping trivalent lanthanide ions into the Ca site in the chemical formulae of Ca1−y Rey MnO3 (Re = La, Pr, Nd, Lu, Sb, and so forth) [1,6] These materials are suitable for high-temperature thermoelectric energy conversion in power generators and refrigerators (known as thermoelectric coolers or Peltier coolers) For Me = ∗ E-mail: scyu@chungbuk.ac.kr; Tel: +82-43-261-2269; Fax: +8243-275-6415 La, Pr, and Nd, Me-doping at high concentrations with y = 0.5 – 0.8 enriches remarkably their electrical and magnetotransport properties, particularly the colossal magnetoresistive effect (CMR) [7,8] This further widens their application range in electronic and spintronic devices, including reading/writing heads, sensitive sensors, and magnetoresistive random access memories [9] Concerning Ca1−y Pry MnO3 , the coexistence of charge-ordering (CO) and antiferromagnetic states has been found as y = 0.5 [8,10] Babushkina and co-workers discovered that the CO state was suppressed strongly when Ca0.5 Pr0.5 MnO3 was doped with Ru, where Ru substitutes for Mn [4, 11] and acts as an electron dopant Also, the incorporation of Ru ions in Mn sites favors the formation of the FM phase [10] Such a feature was found in Ru-doped Sr0.5 Pr0.5 MnO3 [11] Clearly, the Mnsite doping by Ru not only changes the carrier density, the bond angle Mn-O-Mn and the bond length MnO but also leads to interesting magnetic interactions taking place between Mn and Ru ions To get more insight into this problem, we chose antiferromagnetic Ca0.85 Pr0.15 MnO3 (a Mn4+ -rich compound) as a host lattice and doped it with Ru in the chemical formula Ca0.85 Pr0.15 Mn1−x Rux O3 (x = 0.04 and 0.08) The synthesized samples were then studied to obtain detailed structural and magnetic properties -1568- Influences of Ru-doping on the Magnetic Properties of Ca0.85 Pr0.15 Mn1−x Rux O3 – T L Phan et al -1569- Fig (Color online) XRD patterns of polycrystalline Ca0.85 Pr0.15 Mn1−x Rux O3 compounds prepared by using conventional solid-state reaction Fig (Color online) Temperature dependences of the magnetization for Ca0.85 Pr0.15 Mn1−x Rux O3 under an applied field of 150 Oe measured after zero-field cooling II EXPERIMENTAL DETAILS are 0.645 ˚ A and 0.53 ˚ A, respectively Under such circumstances, two possibilities exist: Three polycrystalline samples of Ca0.85 Pr0.15 Mn1−x Rux O3 (x = 0, 0.04, and 0.08) were prepared by using conventional solid-state reaction High-purity powders of CaCO3 , Pr2 O3 , MnO2 , and RuO2 were combined in nominally stoichiometric quantities and were ground well by using an agate mortar and pestle These mixtures calcined at 1000 ◦ C for 24 hrs were then pressed into three pellets under a pressure of about 5000 psi by using a hydraulic press The pellets were then annealed in air at 1180 ◦ C for 24 hrs The final products obtained were checked for crystal structure by using a Brucker D5005 diffractometer working with an X-ray Cu-Kα1 radiation source (λ = 1.5406 ˚ A) Magnetic measurements for various the magnetic fields and temperatures were performed on a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM) III RESULTS AND DISCUSSION Figure shows X-ray diffraction (XRD) patterns for Ca0.85 Pr0.15 Mn1−x Rux O3 (x = 0, 0.04, and 0.08) prepared by using the solid-state reaction method Detailed analyses based on the XRD profiles reveal that the samples had a single phase with an orthorhombic structure and with no additional XRD peaks from the initial powders The lattice constants determined for Ca0.85 Pr0.15 MnO3 (i.e., x = 0) are a = 5.311 ˚ A, b = 5.321 ˚ A, ˚ and c = 7.540 A They slightly increase with increasing Ru doping from x = 0.04 to 0.08 (a = 5.319 ˚ A, b = 5.321 ˚ A, and c = 7.545 ˚ A for x = 0.08) This is due to the fact that the Ru ions that substituted for Mn ions in Ca0.85 Pr0.15 Mn1−x Rux O3 had oxidation states of 4+ and 5+ [3, 4, 10, 11] The ionic radii of Ru4+ and Ru5+ are 0.62 ˚ A and 0.565 ˚ A while those of Mn3+ and Mn4+ (i) Ru4+ tends to substitute for Mn3+ and Ru5+ substitutes for Mn4+ because of the agreement in their ionic radii For a small amount of Ru doping with x = 0.04 - 0.08, the Mn4+ concentration is decreased, and more Mn3+ ions are introduced The substitution of Ru5+ for Mn4+ is thus decreased, promoting the substitution of Ru4+ for Mn3+ (ii) The charge transfer Ru4+ + Mn4+ → Ru5+ + Mn3+ occurs because of doping Ru The average ionic radius of Ru5+ and Mn3+ (0.605 ˚ A) is greater than that of Ru4+ and Mn4+ (0.575 ˚ A) Both the above possibilities lead to the lattice constants of Ca0.85 Pr0.15 Mn1−x Rux O3 (x = 0.04 and 0.08) being greater than those of Ca0.85 Pr0.15 MnO3 However, X-ray absorption and neutron diffraction analyses revealed the second case is usually observed in Ru-doped perovskite manganites [4,11,12] In an attempt to understand the influences of the coexistence of Mn3+ , Mn4+ , Ru4+ , and Ru5+ ions on the magnetic properties of Ca0.85 Pr0.15 Mn1−x Rux O3 , we have measured the magnetization variation with respect to the temperature and magnetic field Figure 2, it plots the temperature dependence of the magnetization, M (T ), for the samples under an applied field of 150 Oe after zero-field cooling One can see that the M (T ) curves exhibit maxima at a temperature of Tm ≈ 102 K for x = and 122 K for x = 0.04 and 0.08, which are associated with the AFM-FM phase transition [12] The Curie temperatures (TC ) obtained from the minima of the dM /dT curves for x = 0, 0.04, and 0.08 at temperatures T > Tm are 107, 135, and 180 K, respectively At temperatures higher than T C , the samples with x = 0.04 and 0.08 enter the paramagnetic (PM) region Meanwhile, x = is still in the AFM regime until T ≈ 145 K (see the inset in Fig 2) The increase in TC with increas- -1570- Journal of the Korean Physical Society, Vol 61, No 10, November 2012 Fig (Color online) Hysteresis loops for x = measured at various temperatures above 90 K The inset shows the coercivity Hc versus temperature Fig (Color online) Hysteresis loop for x = 0.04 measured at various temperatures above 90 K The inset shows Hc as a function of temperature ing Ru content proves that the development of the FM phase is due to additional contributions of ferromagnetic interactions related to the Ru ions, in good agreement with previous reports on Ru-doped Sr0.5 Ca0.5 MnO3 [10], Sr0.5 Pr0.5 MnO3 [11], and SrMnO3 [12] Comparing the magnetic features of our samples with each other, we found that the phase-transition region of x = is very sharp that of x = 0.04 is quite smooth, and that of x = 0.08 has a hump at ∼150 K It is assigned the above phenomena to be due to the magnetic inhomogeneity inside the samples caused by the Ru dopants The magnetic natures of the AFM, FM and PM phases existing in Ca0.85 Pr0.15 Mn1−x Rux O3 can be further characterized by considering their hysteresis loops, M (H) curves, recorded at various temperatures starting from 90 K in the magnetic field range of – 10 kOe For Ca0.85 Pr0.15 MnO3 (the x = sample without Ru dopants), three factors contribute to the M (H) curves at temperatures T < 110 K (see Fig 3): (i) The first one associated with FM interactions causes the hysteresis loops in the field range from −4 to kOe; (ii) the second is due to AFM interactions, leading to a splitting of two lines at fields beyond (4 kOe (as measured at a given temperature); and (iii) the other is PM contributions due to paramagnetic centers, such as isolated ions and/or lattice defects At temperatures T > 110 K, there is a narrow loop (nearly parallel) in the M (H) curves, indicating a remarkable decline of the FM interaction Only AFM and PM contributions persist until T ≈ 150 K Such features agree well with those recorded from M (T ) for x = 0, a shown in the inset of Fig Notably, the coercive field Hc obtained from the M (H) curves also decreases rapidly for magnetic fields from 670 Oe to ∼170 Oe for T > TC = 107 K, (see the inset of Fig 3) We believe that in the x = sample, AFM interactions associated with the superexchange pairs of Mn4+ -Mn4+ and Mn3+ -Mn3+ are dominant There is also a random distribution of FM clusters due to Mn3+ - Mn4+ pairs The magnetic moments of the Mn ions may be governed by a locally anisotropic field generated from the FM clusters at temperatures below 102 K However, at temperatures above 102 K, as the thermal energy is high enough to suppress the FM coupling, the magnetic moments of Mn ions would be governed by the external applied field The impact change of magnetic moments from a local field to an external one leads to the maximum in the zero-field-cooled magnetization of the x = sample Furthermore, because FM clusters are included in the AFM host lattice, the competition between AFM and FM interactions, which take place around cluster boundaries, can lead to magnetic frustration Concerning the x = 0.04 sample (TC = 135 K), Fig shows some representative M (H) curves at various temperatures Different from the previous sample, the main contribution to these curves is the FM phase At temperatures T > 122 K, the saturation magnetization Ms gradually decreases, and the system enters the PM region for T > TC The decrease in Ms at temperatures T < 122 K is due to AFM interactions The variation of Ms is thus similar to that of M (T ) shown in Fig Based on the M (H) curves, we also determined the temperature dependence of Hc , see the inset of Fig Hc (= 335 Oe at 95 K) appears to decrease with increasing temperature from 95 to ∼130 K This is due to the suppression of the FM order under the thermal energy The slight fluctuation of Hc at temperatures above 130 K is assigned to an appearance of Ru-related magnetic phases, as discussed below Assessing in detail the magnetic properties of x = 0.04, we considered the isothermal magnetization curves shown in Fig 5(a), which were recorded around the FMPM phase transition T C in temperature increments of K Arrott plots of M versus H/M [13] will be separated into two characteristic regions with T < 135 K being associated with the FM region and T > 140 K being associated with the PM region, see Fig 5(b) The Influences of Ru-doping on the Magnetic Properties of Ca0.85 Pr0.15 Mn1−x Rux O3 – T L Phan et al -1571- Fig (Color online) Hysteresis loops for x = 0.08 measured at various temperatures above 90 K The inset shows Hc versus temperature Fig (Color online) (a) Isothermal-magnetization curves of x = 0.04 recorded around the phase transition temperature (b) The performance of Arrott plots, M -H/M (c) Ms (T ) and χ−1 (T ) data are fitted to Eqs (1) and (2), respectively; the critical parameters are indicated FM-PM phase transition temperature thus lies between 135 and 140 K We also find that the slopes of the H/M versus M curves (not shown) in the vicinity of the phase transition are positive This indicates that the x = 0.04 sample exhibits a second-order magnetic phase transition according to the Banejeer criterion [14] It means that the spontaneous magnetization (Ms ) and the inverse initial susceptibility (χ−1 ) obey the asymptotic relations [15] Ms (T ) = M0 (−ε)β , ε < 0, γ χ−1 ε > 0, (T ) = (h0 /M0 )ε , (1) (2) where M0 and h0 are the critical amplitudes, and ε = (T – TC )/TC is the reduced temperature The critical exponents β and γ are associated with the exponents of the Ms (T ) and the χ−1 (T ) curves, respectively The features of the Arrott plots reflect that the values of β and γ are close to those expected from the mean-field (MF) theory (where β = 0.5, and γ = 1.0) [15] We used the Arrott-Noakes method [16] to determine Ms (T ) and χ−1 (T ) from the data in Fig 5(b) The fittings of Ms (T ) and χ−1 (T ) to Eqs (1) and (2), respectively, introduce β = 0.478 ± 0.009, γ = 1.252 ± 0.025, and TC = 138.3 K, as shown in Fig 5(c) Obviously, the TC value obtained from this technique is in good agreement with that obtained from M (T ) While our β value is close to that of the MF theory (with β = 0.5), γ is close to that of the tricritical MF theory (with γ = 0.25) [17] The deviation in β value compared to the MF theory demonstrates the presence of short-range ferromagnetic interactions Here, the short-range ferromagnetism is assigned to AFM-interaction pairs, such as Mn4+ -Mn4+ and Mn3+ -Mn3+ , besides the dominant FM interaction pair of Mn3+ -Mn4+ Though the system goes to the PM region, FM clusters still persist at temperatures above T C , making our γ = 0.25 value different from the γ = 1.0 of the MF theory This can explain why there is the fluctuation of Hc at temperatures T > 130 K, as shown in the inset of Fig The hysteresis loops of the last sample with x = 0.08 recorded at several temperatures are shown in Fig Similar to the tendency observed in the x = 0.04 sample, the FM phase continuously develops with increasing Rudoping content in Ca0.85 Pr0.15 Mn1−x Rux O3 Observing carefully, one can see clearly a remarkable difference in the shapes of the hysteresis loops at temperatures T > 140 K This is more visible in the variation of Hc graphed in the inset of Fig At ∼150 K, Hc reaches to a maximum value, coinciding with the temperature of the hump observed in M (T ) for x = 0.08 in Fig A lowering of the temperature below 120 K leads to an enhancement of Hc Because complicated variations of Hc , we believe that magnetic inhomogeneities and multiphases exist in the compound An explanation for the variation in the behavior of the Hc data for the samples with x = 0.04 and 0.08 may be based on the coexistence of Mn3+ , Mn4+ , Ru4+ and Ru5+ ions, and the charge transfer of Ru4+ + Mn4+ → Ru5+ + Mn3+ as increasing Ru-doping concentration Among these, Mn4+ -Mn4+ , Mn3+ -Mn3+ and Ru4+ Ru5+ pairs are AFM while Ru5+ -Ru5+ and Mn3+ -Mn4+ pairs are FM [2, 18] Other pairs related to Mn-Ru interactions are suggested to be AFM [2] For the case of x = 0.04, the additional presence of Mn3+ and Ru5+ ions in Ca0.85 Pr0.15 MnO3 (i.e., x = where the Mn4+ -1572- Journal of the Korean Physical Society, Vol 61, No 10, November 2012 concentration is dominant, as mentioned above) favors the formation of Mn3+ -Mn4+ FM pairs The size of FM clusters thus increases, and they become the dominant phase The Hc value associated with this phase gradually decreases with increasing temperatures up to 130 K Meanwhile, the FM interaction due to Ru5+ Ru5+ can be considered as a secondary FM phase, which causes a fluctuation in Hc at temperatures T > 130 K, as shown in the inset of Fig It should be noticed that the Mn3+ -Mn4+ FM phase of the x = sample (Ca0.85 Pr0.15 MnO3 ), which is dominant in the narrow range of 90 ∼ 115 K, is continuously strengthened and widened by the additional appearance of Mn3+ -Mn4+ at x = 0.04 in Ca0.85 Pr0.15 Mn1−x Rux O3 However, an interesting situation occurs in the case of x = 0.08 Besides the continuous development of the main FM phase associated with Mn3+ -Mn4+ , a secondary FM phase associated with Ru5+ -Ru5+ becomes significant as shown by the appearance of the hump at ∼150 K in M (T ) We predict that further increasing the Ru-doping content to x > 0.08 will broaden the FM phase towards higher temperatures The change in interaction is impact related to the FM phases due to Mn3+ -Mn4+ and Ru5+ -Ru5+ pairs causes an interesting variation in the Hc versus temperature curve, as can be seen in the inset of Fig As mentioned above for x = 0.04, the impact of the Ru5+ -Ru5+ FM phase starts from T ≈ 130 K, and that of the Mn3+ Mn4+ FM phase starts at lower temperatures However, the change in the impact for the x = 0.08 sample starts from ∼120 K, where Hc reaches a minimum value The ascendancy of the Ru5+ -Ru5+ FM phase increases with increasing temperature, indicating an interaction competition of Ru5+ -Ru5+ with other Mn- and Ru-related pairs This Ru5+ -Ru5+ FM phase becomes strongest as T = 150 K and will be suppressed at higher temperatures by the thermal activation energy The above results and explanations show that Ru-doping causes the electrical, magnetic and/or magneto-transport properties of perovskite manganites to become more interesting IV CONCLUSION We prepared three polycrystalline ceramic samples of Ca0.85 Pr0.15 Mn1−x Rux O3 (x = 0, 0.04, and 0.08) and then studied in detail their structures and magnetic properties The XRD data revealed that the samples had a single phase with an orthorhombic structure The slight increases in the lattice constants with increasing Ru content indicated the incorporation of Ru4+ and Ru5+ in the Mn sites This leads to the development of a FM phase associated with Mn3+ -Mn4+ and Ru5+ Ru5+ pairs, and with T C = 135 K for x = 0.04 and T C = 180 K for x = 0.08 The change in the impact related to the Mn3+ -Mn4+ and the Ru5+ -Ru5+ FM phases caused interesting variations in the Hc versus temperature and Ru-doping concentration, leading to the differences in features of the magnetic phase transitions for the sam- ples Such features were explained carefully by means of Hc versus temperature measurements and/or analyses of the critical behaviors in the vicinities of the phase transitions ACKNOWLEDGMENTS This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001431), and partly supported by the VNU Science and Technology Project QG-11-02 REFERENCES [1] B T Cong, T Tsuji, P X Thao, P Q Thanh and Y Yamamura, Physica B 352, 18 (2004) [2] S Mizusaki, M Naito, T Taniguchi, Y Nagata, M Itou, Y Sakurai, Y Noro, T C Ozawa and H Samata, J Phys Condens Matter 22, 145601 (2010) [3] M Naito, S Mizusaki, T Taniguchi, Y Nagata, T C Ozawa, Y Noro and H Samata, J Appl Phys 103, 07C906 (2008) [4] R A Ricciardo, H L 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